In the field of magnetic recording, improvement in the performance of the magnetic recording media is required with rapid progress of the technical art, and in particular, magnetic recording media for video use are required to have a high reproduction output for short wavelength recording. In order to attain these requirements, in general, a highly fine ferromagnetic powder is used in the magnetic layer of the magnetic recording medium, or a binder composition has been proposed to satisfy this requirement.
However, the conventional binder compositions which have heretofore been proposed for magnetic layers are improved to a certain point, but no binder composition which has completely satisfactory properties has yet been obtained.
Hitherto, as the binder for the magnetic layer, there have been used various synthetic resins such as vinyl chloride-vinyl acetate copolymers, cellulose resins, polyurethane resins, acrylic resins, vinylidene chloride resins, epoxy resins, phenoxy resins, synthetic rubber resins, polyester resins, etc., singly or in the form of a mixture of two or more of them. In particular, a combination of a polyurethane resin and a vinyl chloride-vinyl acetate copolymer or a combination of a polyurethane resin and a cellulose resin has been preferably used (as described, e.g., in Japanese patent application (OPI) Nos. 40320/84, 135426/82, 41435/83, and U.S. Pat. No. 4,411,957). The term "OPI" used herein means a published unexamined Japanese patent application.
Particularly, a recent tendency is for the ferromagnetic powder for the magnetic layer to be extremely fine for attaining the above-mentioned requirement, and that a ferromagnetic fine powder having a higher coercive force is used so as to improve the smoothness of the surface of the resulting magnetic layer and as to obtain a higher S/N (signal/noise) ratio. However, when the ferromagnetic powder in the magnetic layer is such highly fine grains, the grains of the powder are strongly bonded together because of the fineness of the grains and their magnetism, whereby it becomes extremely difficult to uniformly disperse the powder in a binder. If the dispersion of the ferromagnetic powder in the magnetic layer is insufficient, the uniformity of the magnetic layer, the orientation of the powder in the magnetic layer and the surface properties of the magnetic layer are poor, and as a result, it is impossible to obtain a high S/N ratio. Further, when the ferromagnetic powder is highly fine grains, the specific surface area of the powder increases so that the contact interface between the ferromagnetic powder and a binder solution is thereby increased and a larger energy is required for dispersing the ferromagnetic powder in the binder solution. This problem could partially be solved by improving the function of a mixer machine for dispersing the magnetic layer-forming composition, which, however, is naturally limited. Accordingly, a surfactant having a dispersing capacity has been added to the magnetic layer-forming composition (as described. e.g., in Japanese Patent Publication Nos. 28369/64, 17945/69, and U.S. Pat. No. 3,470,021). However, increase in the amount of the surfactant added for improvement of the dispersing capacity of the composition causes another problem of deterioration of the physical properties of the resulting magnetic layer.
In addition, magnetic recording media have a problem of static charging, and carbon black or an antistatic agent is generally added to the magnetic layer-forming composition for the purpose of static charge prevention of the resulting magnetic layer (as described, e.g., in Japanese patent application (OPI) No. 38201/77, and U.S. Pat. Nos. 3,545,974 and 3,475,174). This, however, often causes other problems, e.g., decrease in S/N ratio and an adhesion problem by blooming, etc. Recently, improvement of the binder itself has been proposed so as to improve the dispersibility of the ferromagnetic fine powder in the binder and to impart an antistatic property to the binder. For example, a binder having a polar group such as --COOX, --SO.sub.3 X, --OSO.sub.3 X, --PO.sub.3 X.sub.2, --OPO.sub.3 X.sub.2 (in which X represents a hydrogen atom, or an alkali metal atom such as Li, Na, or K) has been developed (as described, e.g., in Japanese patent application (OPI) Nos. 40320/84, 135426/82, and U.S. Pat. No. 4,411,957). This binder has been able to attain a limited improvement, but has insufficient characteristics for running durability. In particular, when a fine ferromagnetic powder having a specific surface area S.sub.BET 40 m.sup.2 /g or more is used with the binder, various problems occur. I.e., the running durability becomes extremely poor and the running tension noticeably increases. The running characteristics are particularly poor when the medium is run under severe high moisture conditions. Accordingly, a new technique capable of overcoming these problems is strongly desired.